In a conventional "H" configuration alternating current power source, a direct current source is applied to a pair of parallel legs, each of which has a pair of series connected switches. A first inductor of a transformer connects the two parallel legs at a point between each pair of series connected switches. The second inductor, the transformer secondary, supplies output power based upon the opening and closing of the switches in each of the series legs. As one switch in one of the series combination upstream of the first inductor feed is closed, the other switch in the series combination is opened, while in the other series combination, the switch upstream of the first inductor connection is open, and the switch downstream switch in the series combination is closed. Once the current is allowed to flow in this configuration, the states of the switches are changed as quickly as possible to cause current to flow.
The problems with this design includes the switching loss, combined with the saturation loss and the current density inefficiency. In order for the switches to operate properly, all of the switches as a practical matter must be open for a short moment before any two can be closed. This is due to (1) making sure that both switches in a parallel leg are not closed at the same time which would cause a short, and (2) to make sure that the silicon controlled rectifier (SCR) switches, if used, have an opportunity to reset. The second, more important source of inefficiency is saturation loss. The losses are proportional to the product of the saturation voltage and the saturation current. The use of a single primary winding limits the output of the transformer and requires control to be limited to the temporal domain.
The control is based upon duty cycle, or the percentage of on time to the total time. Control of the transformer output is based upon a further limitation of the on time, from the upper limit which was already limited to give the switches a chance to clear, etc. Good lower limit control cannot be achieved efficiently since the time for current to begin to flow is not instantaneous. Thus where the pulse is controlled to occupy ten percent of the total allowable time, a ten percent power output will not occur since a significant portion of the pulse time will be spent simply enabling the pulse to rise to an acceptable point. As such, precise control cannot be had based upon percentage of on time, especially at lower percentages of the duty cycle. As the frequency of operation increases, these types of losses become even more severe.
These inefficiencies become even more unwieldy when used in a device for high frequency welding where a significant magnitude output current is generated. Welding and fusing operations require a relatively low voltage and hi current electrical power source for the purpose of fusing or welding two or more metallic surfaces which may be forced against each other under pressure while electric current is passed through the contact junction. The electrical current applied creates heat, which changes the molecular structure of the metallic surfaces in contact with each other and forms a metallic bond with sufficient structural holding strength at the point of contact. Precise control of heat volume and heat time at predetermined speed is required to achieve precise fusing of metals with equal and repetitive weld strength.
In the field of robotic welding, the current source needs to be as close as possible to the welding area for greatest efficiency and to reduce additional heat load from current flowing through cables, especially since the current is so high. The output transformer may be mounted on the robotic arm. To achieve sufficient current with the limitations outlined above, the transformer would have to be unduly large. Carrying a large, heavy transformer or current generator on the robotic arm which moves around the work piece consumes even more energy and requires a robotic arm of increased strength and which is slower, thus causing lower productivity on the line.
Good power transfer should be enabled in all three of high frequency, intermediate frequency and low frequency ranges. Transformers which have the size for significant power output cannot operate at high frequencies with any utilizable efficiency. The natural impedance of a large transformer would not enable it to complete its cycle at high frequency. Most high frequency welding is currently done at a relatively low frequency to enable delivery of sufficient power. It would be helpful to raise the potential operating frequencies to as high as possible to enable welding with greater control of the hot spots and more efficient application of energy to the weld point. This should be possible with an output of from about 1000 amps to about 100,000 amps under ideal conditions.
What is therefore needed is a circuit and device which enables high current at high frequency to be generated. The needed method should include conversion of the standard AC power source available in most industrial facilities, to a high-speed controllable voltage and current electrical power source. The power source may be programmed to produce a modulated heat by means of modulating the electrical current volume. Even more importantly, the needed circuit and device should provide itself with the capability for monitoring, and feedback to provide precise voltage, or current, or power to the fusing or welding electrodes.